Public cellular networks (public land mobile networks) are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
Traditional analog radiotelephone systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well-known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. These discrete frequency bands serve as channels over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 Mhz. At present there are 832, 30-Khz wide, radio channels allocated to cellular mobile communications in the United States.
The limitations on the number of available frequency bands present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system generally requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of frequency bands available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
The past decades have shown a considerable rise in the deployment of mobile telephony. With a slow start of the analog standards like AMPS, NMT and TACS, mobile telephony has really hit the consumer markets with the advanced digital standards like GSM and D-AMPS. In addition to progress in mobile phone features like size and battery life, much progress has been made at the network side as well. Increasingly dense cell reuse plans have been complemented with hierarchical cell structures, where macrocells cover entire districts, microcells cover smaller parts like streets, and picocells cover very small areas the size of a few rooms. Important for the hierarchical cell structure is that each base station deployed (ranging from macro to pico base stations) is part of the same Public Land Mobile Network (PLMN). When a mobile user wanders from a macrocell to a microcell area, the call can be handed off from a macro base station to a micro base station without the user noticing it. This is particularly true for digital phone systems that apply TDMA: being handed off from one base station to another within a coordinated PLMN for the mobile phone usually only involves the change of a time slot.
In order to facilitate the network to reroute the call from one base station to another with the PLMN, the mobile phone performs measurements which are reported to the base it currently connects to using a fast or slow associated control channel (FACCH or SACCH). The mobile measurements help the base station controller of the PLMN in finding the best alternative base station on which the call can be continued.
Recently, private radio communications networks for residential and business areas are being developed that use the same air-interface as the public cellular network, but do not form an integrated part with the overlaying public cellular network. In this sense, these private systems are not micro or pico networks since there is no direct connection between these private systems and the public cellular network. For example, for residential usage, private base stations can be used as described in U.S. Pat. No. 5,428,668 which only connect to the PSTN. In business or office networks applying a Private Branch Exchange ("PBX"), radio base stations belonging to the same private network communicate with each other, but none of them communicates directly with the overlaying PLMN. Because the private radio systems have only a limited range, a call drop will result when the mobile user moves out of the coverage range, unless the overlaying public cellular network can take over the call. Therefore, a handover from the private mobile network to the public cellular network is highly desirable. However, because there is no coordination between the private radio system and the PLMN, problems are encountered in achieving a handover to the PLMN from the private network.